Although the discussion in this specification applies to all optical filters (including any lens, mirror, and any other optical filter), such discussion will sometimes include specific references to the lenses of sunglasses for simplicity. Sunglass lenses are an important embodiment of the inventive optical filter, though by no means is such embodiment the only one contemplated by the inventor.
The common gray, green or brown tinted sunglass is usually a colored glass that absorbs a significant amount of radiation in both the visible and ultra-violet spectra. It is very difficult to create a tinted glass that has near neutral visible radiation transmission qualities and also absorbs ultra-violet or infrared radiation (or both ultra-violet and infrared radiation).
It is well known that clear glass (e.g. BK-7) will transmit the entire visible spectrum, and that a coating may be applied to clear glass in order to reflect certain wavelengths of light. Any wavelength which is completely reflected from such coated glass will not be transmitted. Therefore, these wavelengths will be taken out of the color spectrum when viewing natural objects through the glass.
It is also known that two quarter-wave stacks, where all layers comprising each stack have the same quarter-wave optical thickness, deposited upon a transparent substrate, may be used as a short-wave-pass (SWP) filter. It is conventional to adjust the optical thickness of each stack so that the visible wavelengths (400 nm to 680 nm) will not reflect and the near-infrared (NIR) wavelengths (700 nm to 1100 nm) will totally reflect from the coated substrate. It is also known that a substrate (commonly glass) that absorbs a percentage of the visible wavelengths and all ultra-violet radiation may be employed as a sunglass, and could be coated by a quarter-wave stack of the above-described type. This sunglass will absorb all UV solar radiation and reflect all NIR solar radiation and will allow only a percentage of the total visible spectrum to pass through to the eye.
It is conventional that the double quarter-wave stack may have more than 25 layers whose thickness is optimized so that a very small part of one (or both) of the colors blue and red (on the edges of the visible spectrum) are reflected for aesthetic purposes. Revo.RTM. sunglasses (commercially available through Coopervision, Inc.) are an example of eyeglasses having such a multilayer coating. By reflecting only a small amount of the visible radiation, and only reflecting visible radiation in the wavelength regions where the human eye is least sensitive (in the blue and red), a relatively correct color balance is maintained in transmission. This color balance and the observed reflected aesthetic color can be computed numerically and plotted on a chromaticity chart. Since the glass conventionally used absorbs more blue than red and yellow light, the transmitted color balance tends to be slightly yellow.
If the described multilayer coating were scratched, a visual defect would be observed due to the multilayer interference. However, a thick top layer of fused silica is conventionally used to make most scratches unobservable.
Although such conventional lenses are designed to have desirable optical properties, they are very difficult and expensive to manufacture. Specifically, the design of the prior art lenses require that the coatings must be applied with great accuracy. However, deviations in the manufacturing process commonly cause the reflected wavelengths to vary. If the variation is such that the reflected range is out of the visible region (a common occurrence), the lens will not be properly colored in reflection. If the shift moves the reflected range further into the visible region (another common occurrence), too much visible radiation will be reflected and the transmitted color balance will be lost. This latter result is particularly a problem if red light is reflected so that the wearer might not be able to detect a red traffic light.
Also, since the substrate of this type of prior art lens has a curved surface, a green ring is typically observable (when reflecting red light) near the edge of the lens because the coating's spectral curve shifts toward shorter wavelengths with increasing incidence angle.
More simplistic coating designs are utilized on other commercially available sunglasses. These simple designs typically use a basic five to ten layer broad band anti-reflection coating (AR). These coatings reflect some part of the visible spectrum for aesthetic purposes. However, they do not simultaneously preserve neutral transmitted color balance and block substantially all infrared light. If one were to integrate to find the area under the reflectance spectrum of one of these filters (over the visible wavelength band) the total area would be more than one would obtain if one performed similar integration of a reflectance curve characterizing the inventive filter. The quality of the color balance is accordingly degraded in the conventional coating design. Although the manufacturing complexity of this simple conventional coating design, and the cost to manufacture filters embodying such design, is relatively low, the optical performance of filters embodying such simple coating design is compromised.
It has not been known until the present invention how optical filters (used as sunglasses) having a selected aesthetic reflected color and a neutral transmitted color balance may be repeatably and economically produced.